List::Util is a dual life module, so you can download newer version from CPAN. So in this case, people with older versions of Perl could just update List::Util to 1.44 or higher. List::Util 1.44 works on Perl versions as old as 5.8.1 and perhaps older.

Generally speaking, I'd say when releasing code, it's my responsibility to indicate what modules it depends on plus the minimum version of Perl for the features *I* am using. I don't think it's a good idea to also try to figure out the minimum version of Perl recursively based on all my dependencies because the authors of those dependencies might release a new version tomorrow which raises or lowers that.